1. Field of the Invention
The invention relates to a process for preparing dimethyl propanephosphonate, characterized in that dimethyl phosphite is reacted with propene in the presence of a free-radical former at a reaction temperature of 60 to 95° C., the half life of the free-radical former being 0.1 to 20 min at the reaction temperature.
2. Brief Description of the Prior Art
Alkanephosphonic dialkyl esters have a broad spectrum of use; for example they serve in plastics as non-reactive flame retardants and plasticizers, and they are used as aids in textiles and paper. In addition, they are used in the chemical industry as intermediates, extraction media for metals and as additives for lubricants. Dimethyl propanephosphonate, in particular, according to DE-A-4418307 is a good flame retardant for polyurethane plastics.
A multiplicity of processes are known for preparing alkanephosphonic dialkyl esters. Because of its efficiency, free-radical addition of dialkyl phosphites to olefins is frequently used (Houben-Weyl, Methoden der Organischen Chemie [Methods in Organic Chemistry] 4th edition, volume XII/1, Stuttgart 1964, pp. 463–467; volume E2, Stuttgart 1982, pp. 351–353).
A particular technical implementation of this process is described in DE-A-2 043 520 with respect to the preparation of dimethyl propanephosphonate. The process according to DE-A-2 043 520 permits the preparation of ethanephosphonic and propanephosphonic esters in yields of greater than 95%, without telomers being formed. According to DE-A-2 043 520, the reaction of dialkyl phosphites with ethylene or propylene at 130 to 250° C. in the presence of free-radical formers must be carried out in such a manner that the gaseous olefin is added to the dialkyl phosphite present to the extent that it is reacted.
DE-A-1 963 014 describes required temperatures of 150 to 195° C. in the reaction of dialkyl phosphites with olefins. In this temperature range, all known free-radical formers should be suitable for the reaction.
A disadvantage of the above-described processes in the synthesis of dimethyl propanephosphonate is that, in the synthesis, in addition to dimethyl propanephosphonate, there is also formed 0.1 to 1.0% by weight of trimethyl phosphate which is undesirable. Trimethyl phosphate is included among the work-area substances suspected of carcinogenic activity (Deutsche Forschungsgemeinschaft, “Maximale Arbeitsplatzkonzentrationen und Biologische Arbeitsstofftoleranzwerte—Mitteilung 37” [Maximum workplace concentrations and biological agent tolerance values—Part 37], Wiley-VCH, Weinheim 2001, p. 110). In the European Union countries, preparations containing a carcinogenic substance in amounts greater than or equal to 0.1% must be appropriately labelled (directive 1999/45/EG, Annex II, Chapter 6 (previously 88/379/EWG), transposed into German law by the GefStoffV [Dangerous substances regulation]). This labelling obligation is a considerable competitive disadvantage. Trimethyl phosphate is therefore an unwanted byproduct.
Lowering the reaction, temperature does not necessarily lead to a high yield of alkanephosphonic dialkyl ester as can be implied from U.S. Pat. Nos. 2,478,390 and 4,003,720, where in the free-radical addition of dialkyl phosphites to olefins at temperatures of 60 to 150° C., telomers are predominantly formed. These telomers are unsuitable for use as flame retardant. According to U.S. Pat. No. 2,478,390, the reaction of diethyl phosphate with ethylene at 80–115° C. gives only 42% diethyl ethanephosphonate, while the remainder of the reaction product consists of high-boiling telomers.
The object underlying the invention was to provide a further improved process for preparing dimethyl propanephosphonate. As would be appreciated, the object is made more difficult by the fact that the boiling points of trimethyl phosphate and dimethyl propanephosphonate only differ by about 2° C., so that separation of the two substances by distillation can only be achieved with considerable expenditure.